Transponder units are currently available for sensing the operating temperature of a power transmission line and for monitoring the current through the transmission line on a continuous basis. The information is transmitted by means of a radio frequency transmitter to a local receiver for evaluation as to the condition of the transmission line under periods of heavy loading. The transponder unit generally comprises a closed loop of a metallic tubular material which includes the radio transmitter and power supply elements. The loop housing is attached to the transmission line by means of a housing clamp which must be connected to the transmission line when the line is de-energized. The radio frequency antenna generally comprises a linear extent of antenna wire wrapped around the transmission line and attached to the line by means of an insulated antenna clamp. The power supply and transmitter circuit comprises an analog circuit coupled with a modulator connected to the thermal and current sensors for obtaining the necessary electrical and thermal data. The conductor temperature is obtained from a thermocouple mounted opposite from the antenna. The modulator is connected to a standard radio frequency transmitter and antenna for transmitting the electrical and thermal data to a nearby receiver. The power supply within the circuit is powered by means of the electricity generated within the loop housing by means of the varying magnetic field surrounding the power transmission line. Since the tubular housing provides the means for intercepting the surrounding magnetic field or air flux for generating electric power, the closed loop must be in the order of a few feet in circumference in order to provide enough electrical energy from the magnetic field to power the necessary electronic circuit components in the unit. The use of a radio frequency transmitter requires a radio frequency antenna several feet long for efficient transmission to a receiver located as far as fifty feet from the power transmission line.
The purpose of this invention is to provide a transponder unit for sensing the current through a transmission line, as well as the transmission line temperature, without having to de-energize the transmission line during installation, removal and repair. The improved transponder unit is more compact in view of its improved magnetic power loop and ultrahigh frequency transmitter.